Astrophile: A handy guide to planetary parking spots

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Objects: Empty orbital slotsAvailability: Good

Need somewhere to park your planet? You won't have to circle the galaxy for long: up to two-thirds of planetary systems have empty spaces where an extra world could comfortably reside.

The gravitational tug-of-war between a star and its orbiting planets means that the worlds must be spaced at particular distances or their orbits become unstable. An overcrowded system will cause planets to wobble around until some tangle, collide or are ejected.

Astronomers' current understanding of planetary formation suggests that most – if not all – stable systems should be filled to capacity. But that doesn't always seem to be the case.

"In the solar system, we know that's not quite true, because we know that in between Mars and Jupiter you could put another planet," says Sean Raymond at the Laboratory of Astrophysics of Bordeaux in France. In fact, some astronomers think we started out with more worlds, but gravitational jostling with Jupiter caused some to be ejected about 4 billion years ago, leaving the empty slot.

Spare room

Julia Fang and Jean-Luc Margot at the University of California, Los Angeles, wanted to find out whether other planetary systems are full, or if they also have unoccupied but stable orbits in between their planets.

The pair simulated millions of systems with two, three or four planets in a variety of orbital configurations and compared their models to real star systems seen by NASA's Kepler space telescope. This told them which of their model systems are spaced right to be stable.

The scientists checked whether any of these systems had orbital slots going spare. "We attempted to stick an additional planet in between two existing planets," says Fang. They then modelled how the orbits evolved over 100 million years, to see if a new planet caused a collision or ejection.

Conspicuous absence

Fang and Margot discovered that about a third of the stable two- and three-planet systems they modelled would go haywire if they added a planet, rising to nearly half for four-planet systems. That means the remaining majority of systems had exploitable stable zones, although that proportion could be revised downwards as Kepler and other planet hunters make more discoveries.

Pinning down the stable spaces between known exoplanets might be useful for finding otherwise invisible worlds, says Raymond. "You can say, 'We think there should be a planet on this orbit, go look for it'," he says.

And if a system has a truly unoccupied slot, could a sufficiently advanced civilisation build its own artificial planet and park it in orbit? "Gravitationally it would certainly work out, I'm just not sure about the logistics," says Fang. Perhaps it's time to bring legendary planet designer Slartibartfast out of retirement.

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